1. Field of the Invention
This invention relates to frequency synthesizers, particularly to the voltage-controlled oscillator (VCO) of a frequency synthesizers for radio transceivers.
2. Brief Description of Related Art
In wireless communications system, frequency synthesizers are used to generate stable local oscillator (LO) frequencies with very high accuracy. A frequency synthesizer is usually a phase-locked loop (PLL) whose simplified diagram is shown in FIG. 1. It consists of a reference frequency divider (1/M), a feedback frequency divider (1/N), a phase comparator (PC), a charge pump (CP), a loop filter (LPF) and a VCO. Under phase-locked condition, the frequencies of the two signals at the PC inputs are identical. This forces the VCO frequency (Fout) to be NIM of the input reference frequency (Fref).
Usually the center frequency of a free-running VCO exhibits large variations over process and temperature. In order to generate the desired LO frequency, the frequency tuning range of the VCO needs to be large enough to overcome the above variations. Unfortunately, this leads to a large VCO gain (Kvco), which results in poor phase noise of the PLL.
To increase the tuning range of the VCO without sacrificing the phase noise performance, Welland proposed in U.S. Pat. No. 6,137,372 to use a coarse tuning arrangement in conjunction with fine tuning for the VCO. The VCO is designed in such a way that its frequency can be controlled both discretely through some digital bits and continuously through an analog voltage. The digital control varies the VCO frequency over a large range with coarse steps; while the analog control tunes the VCO frequency over a much narrower range.
Chien proposed in U.S. Pat. No. 6,597,249 a binary search algorithm to find the optimal digital control word. The binary search algorithm greatly reduces the search time compared to a linear search algorithm.